This research is directed toward an understanding of biological assembly processes which involve participation of molecules not found in the assembled structure. Bacteriophage lambda head assembly is used as a model for these processes. Aspects of viral assembly being studied include the participation of the host cell in assembly, protein processing reactions that occur during assembly, and a virus-coded protein that acts catalytically in assembly. In order to understand the assembly processes, the detailed structure of the assembled head will be examined by chemical, immunological and electron microscopic techniques. The chemical structure of the products of an unusual protein fusion reaction that occurs during assembly will be examined. Complexes of proteins that are assembly intermediates will be sought and characterized. A host-coded protein involved in virus head assembly that has recently been identified will be characterized and tested for interactions with virus proteins. An attempt will be made to carry out the earliest steps of virus assembly in vitro.